Intracellular bacterial pathogens are a leading cause of morbidity and mortality in humans worldwide. The intracellular bacterial pathogens Salmonella enterica serovar Typhimurium (S. Typhimurium) and Francisella tularensis (F. tularensis) are a leading cause of inflammatory enterocolitis and the causative agent of tularemia, respectively. S. Typhimurium can infect large numbers of people through contaminated food and water supplies, and were among the first microorganisms used in deliberate bioterrorism attacks. F. tularensis are extremely virulent for humans when aerosolized and have the potential for use as a bioterrorism agent. S. Typhimurium and F. tularensis can kill professional antigen presenting cells to avoid immune clearance and to replicate within mammalian hosts. In vitro studies have shown that S. Typhimurium and F. tularensis kill macrophages and dendritic cells by activating the mammalian cysteine protease Caspase-1. Caspase-1 is the central effector molecule of the inflammasome, a multiprotein complex required for innate immune defense against a number of microbial pathogens. In vivo studies have shown that Caspase-1-deficient mice are more susceptible to S. Typhimurium and F. tularensis infection than wild-type mice, indicating that Caspase-1 is essential for innate immune defense against these intracellular bacterial pathogens. The experiments described in this proposal seek to further characterize the role of Caspase-1 in host response to S. Typhimurium and F. tularensis infection. The first Aim of this proposal will determine which cell type(s) must express Caspase-1 for innate immune defense against S. Typhimurium and F. tularensis. The second Aim of this proposal will determine if Caspase-1 expression by the host regulates induction of protective, T cell-mediated adaptive immunity to S. Typhimurium and F. tularensis. These in vivo experiments are critical to the understanding of host response to infection with Category A and B pathogens and will be conducted as part of a collaborative Program Project Grant, entitled Agents of Bioterrorism: Pathogenesis and Host Defense." All experiments involving F. tularensis will be conducted in collaboration with Project 2 (Furie) and Project 5 (Thanassi), and in vivo experiments involving F. tularensis strains that are virulent in humans will be conducted at the Public Health Research Institute (Perlin). The synergy between the Projects and Core Resources of this Program Project Grant will help illuminate additional aspects of the complex interactions between Category A and B pathogens and their mammalian hosts. RELEVANCE (Seeinstructions): The experiments described herein will focus on the mammalian immune response to the potential bioterrorism agents Salmonella enterica serovar Typhimurium (S. Typhimurium) and Francisella tularensis (F. tularensis). The proposed experiments should provide new insight into how these bacteria evolved to overcome the immune system, which may lead to the development of novel therapeutics or vaccines for treatment and prevention of human disease caused by S. Typhimurium and F. tularensis.